The maintenance of data processing system transmission security and integrity poses ever expanding problems due to the ever increasing utilization of telephone and other conventional commercially available transmission facilities for binary form data transmission and to the increased opportunities for interception, error introduction and for transmission modification. One area of growing concern is the assuring of the exact duplication of the transmitted and received message information and particularly in those instances where such iformation transmission constitutes an intermediate step in a sequence of separate but related and interdependent data processing operations.